Generally, the shadow mask of the braun tube (illustrated in FIG. 1) has the color selection function. This shadow mask is manufactured in the following manner. That is, as shown in FIG. 2, a plurality of tiny holes are formed in a cold rolled steel sheet by applying a photo etching method. Then a final decarburization is carried out, and then, a press-forming is carried out.
For the shadow mask, a high purity steel is required, and at the same time, it is important that the carbon content is controlled to a certain level. The reason is that when the press-forming is carried out by the cathode ray tube manufacturing company, the carbon content is closely related to the generation of defects. That is, if the yield strength increases due to the increase of the carbon content, then the shape fixability is aggravated. Further, due to the generation of a stretcher strain caused by the solute carbon, a non-uniform deformation is formed, with the result that the sizes of the holes (formed by applying the photo etching process) are varied.
Techniques for solving the above described problems are disclosed in U.S. Pat. Nos. 4,210,843, 4,609,412 and 4,235,752.
First, in U.S. Pat. No. 4,210,843, there is used an IF (interstitial-free) steel in which the carbon content is extremely low, that is, as low as 0.01 wt % (to be called simply "%" below), and Nb is singly added, or Nb and Ti are combinedly added. In this manner, the yield point elongation is eliminated. In this method, however, the expensive alloy element has to be added. Further, the recrystallization temperature rises due to the added element, and the magnetic properties are aggravated due to the inhibition of the grain growth, which is caused by the precipitates.
Meanwhile, in U.S. Pat. No. 4,609,412, the carbon content is regulated to below 0.004% to improve the demagnetization characteristics of the shadow mask. To achieve this, the carbon content is controlled to below 0.008% at the steel manufacturing stage, and the carbon content is lowered down to 0.005% at an intermediate decarburization annealing. In this method, however, an OCA (open coil annealing) is adopted at the intermediate decarburization annealing, and therefore, the treating time is as long as several days. Further, the carbon content is regulated to 0.004% or 0.005% at the cold rolled state, and in the case where the cathode ray tube manufacturing company carries the annealing by stacking the steel sheets, the yield point elongation occurs at the intermediate portion of the stacking (FIG. 4). Therefore, the relevant sheets have to pass the roller leveller, if the forming defects are to be prevented.
Meanwhile, in U.S. Pat. No. 4,235,752, a method is proposed in which the initial carbon content is regulated to 0.01%, and the final decarburization annealing by the cathode ray tube manufacturing company is carried out at a temperature of 650-850.degree. C. In this method, however, the lowering of the carbon content is heavily dependent on the cathode ray tube manufacturing company, and therefore, as in the above described case, the steel sheets stacked in the middle level are liable to show the forming defects. Therefore, the disadvantage exists that the roller levelling has to be carried out before the press-forming.